1. Field of the Invention
This invention relates to the field of electrochemistry. This invention more particularly relates to thermally activated electrochemical cells and still more particularly to thermally activated electrochemical cells including a novel electrolyte system.
2. Description of the Prior Art
Thermally activated electrochemical cells or batteries have been used quite extensively in military applications, such as a power source for arming devices, because of their long shelf life and compactness, and capability of withstanding shock and vibration. Batteries of this type typically include an electrolyte which, under normal storage conditions, is solid and does not conduct electricity. When the battery and/or the electrolyte is heated to a predetermined temperature, as by a built-in pyrotechnic heat source, the electrolyte, upon changing to a molten state, becomes electrically conductive and ionically connects the electrodes to provide the desired electromotive force.
Most of the thermal batteries presently employed in missile systems make use of a lithium chloride-potassium chloride (LiCl-KCl) mixture as the electrolyte, calcium metal as the anode and calcium chromate (CaCrO.sub.4) as the cathodic material. The relatively high melting point of the electrolyte limits the activation of the battery to temperatures above 352.degree. C., and thermal batteries using LiCl-KCl mixtures are generally designed to operate at internal temperatures of between 475.degree. and 550.degree. C.
Nitrate salts have been proposed for use in thermal batteries because of their low melting points. See U.S. Pat. No. 4,260,667 to the applicants and hereby incorporated by reference. For example, potassium nitrate-lithium nitrate (KNO.sub.3 -LiNO.sub.3) mixtures melt at temperatures as low as 132.degree. C. The use of a lower melting electrolyte can shorten a battery's activation time and reduce the weight of heat sources and insulation. Another major advantage is that the molten nitrate can also function as the oxidizer thus simplifying battery construction by eliminating the necessity for the addition of a separate oxidizer such as CaCrO.sub.4. Furthermore, nitrate salts are low-hazard materials, unlike chromates, which are recognized as health hazards (CaCrO.sub.4 has been confirmed as a carcinogen). The high rate discharge of prior nitrate salt-containing battery cells, however, have been limited by both the anode and the cathode.